With the recent development of wireless networking, communication systems based on the Institute of Electrical and Electronic Engineering (IEEE) 802.11 wireless local area network (WLAN) standard have become very popular. The IEEE society, a professional organization, has formulated and promulgated standards, through its standards committees, that are recognized internationally for providing commercial efficiency for communication, power, computing and many other systems that require a set procedure for the effective and efficient use of the systems among several different entities. In this particular case, the standards referred to provide one definitive protocol for all equipment vendors so that the communication equipment from the various manufacturers will interface with one another in the wireless local area network, such as the systems illustrated in FIGS. 1 and 2. Data standards referred to herein may be found in the IEEE 802.11 WLAN standards.
While the data service based on WLAN is extensive, the commercial usage of WLAN to support real-time interactive multimedia service is rare. This is primarily due to the fact that a very high percentage of the currently available IEEE 802.11 cards on the market implement only the contention-based medium access control (MAC) layer mode, i.e. the distributed coordination function (DCF), defined by the IEEE 802.11 standard. As described later, the DCF mode cannot be used to support the real-time application. The IEEE has been aware of this problem, and as a result, they have provided for a polling based MAC layer operational mode for the IEEE 802.11 standard, namely the point coordination function (PCF).
IEEE 802.11 is a standard access technology defined by the IEEE 802.11 working group. The standard 802.11 WLAN supports two structures, the infrastructure mode and the ad-hoc mode. In the infrastructure mode as described in FIG. 2, all data from the Mobile Terminals (MT) h will be sent to an Access Point (AP) a, b, c, d, e, f, and g and the receiving AP will subsequently send the packets to the proper destination. In the ad-hoc mode, the MTs h will directly communicate with each other.
The IEEE 802.11 standard also supports three different physical layer schemes. They are the Frequency Hopping Spread Spectrum (FHSS), the Direct Sequence Spread Spectrum (DSSS) and the Infrared (IR). With the different physical layer technologies mentioned above, up to 11 Mbps is supported. In order to further increase the data rate, the IEEE 802.11a extension is standardized. IEEE 802.11a uses the Orthogonal Frequency Divided Multiplexing (OFDM) as its physical layer modulation scheme supporting a 54 Mbps data rate. Despite the differences between the physical layers, the IEEE 802.11 uses a common MAC layer standard. Therefore, this formatted message frame is the standard in the above IEEE 802.11 physical layer schemes. In this 802.11 MAC standard, two operational modes are supported, the DCF mode and the PCF mode.
As shown in FIG. 3, the DCF mode is a contention based operational mode. A Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism is used within the DCF mode. When a MT h wants to send data, it monitors the wireless channel. If the channel is free, instead of immediately transmitting the data, the MT h will delay for a short period known as the DCF Interframe Space (DIFS) i. After the MT h has delayed for a DIFS period i, the MT h monitors the channel once again. Only if the channel is free will the MT h transmit the packet. On the other hand, if the channel is busy, the MT h will back-off for a random period of time before attempting to transmit again. Since each MT h competes for the medium, the DCF mode is called a contention-based operational mode. FIG. 3 makes clear that the MTs h operating in the DCF mode transmit randomly. As a result, no systematic frame boundaries are defined for the MTs h using the DCF mode, and the jitter between two sequential data frames cannot be predicted. Hence, this makes the DCF mode inherently unsuitable for a reliable multimedia service.
In order to support a delay sensitive application such as voice, the IEEE 802.11 standard provides another MAC layer operation mode, the PCF mode. FIG. 4 illustrates a contention free period where the PCF, a polling based operation mode, is utilized. Before any AP transmits data, the AP will monitor the assigned channel for other ongoing communications. If the channel is free, similar to DCF, the AP will delay for a short period of time known as the PCF Interframe Space (PIFS) j as shown in FIG. 5. Since a PIFS time period j is shorter than a DIFS i, an AP operating in the PCF mode, will transmit before the time elapses for any of the DCF devices, thereby securing the transmission slot. The period when the PCF operates is known as the Contention-Free Period (CFP) and it is configurable using the CF_Max_Duration parameter defined by the IEEE standard. Each AP will generate the CFP at a fixed rate known as the Contention-Free Repetition Rate (CFPRate). From the end of the current CFP to the beginning of the next CFP, the AP will operate in the DCF mode. This period is called the CP (Contention Period) k and is illustrated in FIG. 4. The sum of the CFP and CP periods is called Contention Free Repetition Interval and is illustrated in FIG. 6. The Contention Free Repetition Interval is also known as a superframe.
Each AP maintains a polling list. During each CFP, the AP polls each member on the polling list at least once using the CF−Poll+Data m message frame. The MT who receives the CF−Poll+Data m message will transmit an acknowledgement and the reply data using the ACK+Data n message frame. The space between each data frame is the Short Interframe Space (SIFS) o, which is shorter than both the DIFS i and the PIFS j time periods as illustrated in FIG. 5. At the beginning of each CFP period, see FIGS. 4 and 6, a beacon p will be transmitted on an interval basis defined as Tb.
There are two ways to end a CFP period. The first is when the CF_Max_Duration time has elapsed. The second way to end the CFP period is when the AP transmits a CF_END q message after it has polled all the client members on the polling list. Since the PCF mode provides a connection-oriented service with these delay boundaries, it will support a multimedia service.
A multi-hop network is a type of wireless network whose nodes relay informational packets to other nodes that are out of the communication reach of the central node, from where the transmission process originates, in order to extend the coverage area of the network. The use of the IEEE 802.11 based multi-hop network has real world merits. For example, cell phone usage currently interferes with the medical equipment in hospitals, and subsequently cell phone usage is banned there. The 802.11 WLAN, on the other hand, does not interfere with medical equipment. Therefore, it would be meaningful to use the 802.11 WLAN to deliver voice service, and with the use of a multi-hop network, the coverage area could be extended. However, the simple use of the current PCF mode, standardized in the IEEE 802.11 standards, will not support voice service over such a multi-hop network. This topic is more thoroughly covered in the IEEE 802.11 WLAN standards. Accordingly, there is a need for an improved WLAN protocol that allows for improved multi-hop service.